I had scales under my hives the last few years. It looks like the schedule in my part of Hudson Valley, NY is something like:
March/April: some pollen, almost enough nectar to break even but not quite, stuff like skunk cabbage and maple blooming
May 7 -- June 1: main flow, mostly russian olive or autumn olive (I don't know the difference), watery clear nectar
June 1 -- July 1: kind of break even, with yellow sweet clover, white clover, some trees, some wild berry bushes
July 4-ish: freaky strong and short flow, no idea what it is but it hits hard and fast, theories include linden and alfalfa farms *shrug*
late July / early August: kind of break even on asters and random stuff
late August: decent flow, stinky goldenrod, milkweed, strong dark honey
September: kind of break even on asters and stuff
So it seems like there's no real "dearth" here where you'd need to feed the bees, just break-even periods between sporadic flows. I don't know much about the tree flows because I can't see the bees up there. It's much easier to notice them down near the ground.